1. Field of the Invention
This invention relates to a thermoplastic elastomer composition for a powder material used for slush molding, as to form a skin. More specifically, the invention relates to a thermoplastic elastomer composition containing a hydrogenated block copolymer compatible with a polypropylene resin.
2. Background Art
It is known to form skins on automobile interior parts, such as instrument panels, console boxes, door trims, etc. through a powder slush molding processing using soft powder materials. It is known to use plasticized polyvinyl chloride resin for this purpose. Powder slush molding is desirable in that a skin with a soft texture can be produced as well as one with flexible design characteristics. For example, the skins can be made to simulate leather wrinkles and/or stitching.
Powder slush molding differs from injection molding and compression molding processes in that the powder materials are not placed under pressure during the forming process. Thus, the powder material, once melted, is required to flow readily to produce a uniform thickness oft times over a complex metal mold. The melt viscosity also must be low so that the powder deposited against the mold can melt and form a film without being placed under pressure. It is also important that the skin be easily released from the mold after it is formed and cooled.
In one conventional process, as disclosed in Japanese unexamined patent application publication No.7-82433, pulverized polypropylene resin and a specific thermoplastic styrene elastomer are mixed in a ratio of 70/30 to 30/70 by mass. The thermoplastic styrene elastomer is selected from styrene-ethylene butylene-styrene block copolymers containing 20 mass % or less styrene, styrene-ethylene propylene-styrene block copolymers containing 20 mass % or less styrene, and hydrogenated styrene-butadiene rubber containing 20 mass % or less styrene, so that the composition is compatible with polypropylene resin and suitable for powder molding.
In Japanese unexamined patent application publication No. 10-30036, a thermoplastic elastomer composition for slush molding is disclosed that is an ethylene-xcex1-olefin copolymer, a polyolefin resin, and a hydrogenated diene copolymer. The hydrogenated diene copolymer is prepared by hydrogenating a conjugated diene copolymer or a conjugated diene-vinyl aromatic hydrocarbon random copolymer containing 25 mass % or less vinyl aromatic hydrocarbon. The hydrogenation degree is at least 70%.
In Japanese patent publication No. 2973353, a thermoplastic elastomer composition for slush molding is disclosed with a thermoplastic elastomer selected from ethylene-ethylene butylene-ethylene block copolymers and ethylene-octene copolymers blended with a polypropylene resin and a hydrogenated styrene-butadiene random copolymer. The mixture is pulverized.
In the above styrene-ethylene butylene-styrene block copolymers, there is no disclosure of hydrogenated block copolymers having a main block consisting of butadiene monomer units which, before hydrogenation, has 62 mol % or more 1, 2 bonds on average.
Problems arose in the past where the elastomers were not compatible with the polypropylene resin. As an example, when ethylene-propylene rubber (EPR), ethylene octene copolymer (EOR), styrene-ethylene butylene-styrene block copolymer containing 25 mass % or more styrene, styrene-ethylene propylene-styrene block copolymer and/or hydrogenated styrene-butadiene rubber were blended, the elastomer components did not uniformly disperse into polypropylene resin. As a result, the compositions often were not as formable as desired, had poorer than desired physical properties, and blushed from folding. Further, the physical properties varied considerably with temperature change, thereby making it impractical to use skins made from these compositions for certain applications, such as for storage box covers for air bags.
In one form, the invention is directed to a thermoplastic elastomer composition for a powder material used for slush molding. The thermoplastic elastomer composition consists of a polypropylene resin and 20 to 500 parts by mass of a hydrogenated block copolymer per 100 parts by mass of the polypropylene resin. The hydrogenated block copolymer has a) at least one polymer block A with a primary component that is a vinyl aromatic hydrocarbon monomer unit and b) at least one polymer block B with a primary component that is a hydrogenated butadiene monomer unit. The polymer block B has a hydrogenation degree of at least 90%. The vinyl aromatic hydrocarbon in the hydrogenated block copolymer is present in an amount more than 5 mass % and less than 25 mass %. The polymer block B before hydrogenation contains 62 mol % or more 1, 2 bonds on average. The melt flow rate (MFR) of the thermoplastic elastomer composition is at least 10 g/10 min at 230xc2x0 C. under a load of 2.16 kgf in accordance with Japanese Industrial Standards (JIS) K7210.
In one form, the thermoplastic elastomer composition further consists of 20 to 200 parts by mass per 100 parts by mass of the polypropylene resin of at least one of a) a block and b) a random copolymer of styrene and a conjugated diene with a hydrogenation degree of at least 90%. The styrene is present in the at least one of the a) block and b) random copolymer of styrene and a conjugated diene in an amount more than 14 mass % and less than 50 mass %.
In one form, the thermoplastic elastomer composition further includes 5 to 250 parts by mass of an ethylene-xcex1-olefin copolymer rubber per 100 parts by mass of the hydrogenated block copolymer.
In one form, the polypropylene resin consists of a propylene-xcex1-olefin copolymer having a melting point of 120xc2x0 to 145xc2x0 C. measured with a differential colorimeter at a heating rate of 5xc2x0 C./min.
The thermoplastic elastomer composition may further include 0.02 to 5.0 parts by mass of an organic peroxide per 100 parts by mass of the polypropylene resin.
The thermoplastic elastomer composition may further include a process oil.
In one form, the thermoplastic elastomer composition is freeze-pulverized to produce particles having a size to pass through a sieve not greater than 1.00 mm.
In one form, the thermoplastic elastomer composition is treated by hot-cutting in water to produce particles having an effective mean diameter of 1.00 mm or less.
The invention is also directed to thermoplastic elastomer composition for a powder material used for slush molding, which thermoplastic elastomer composition consists of a polypropylene resin, 20 to 300 parts by mass of a random copolymer of styrene and a conjugated diene per 100 parts by mass of the polypropylene resin, and 20 to 200 parts by mass of at least one of a) a block and b) a random copolymer of styrene and a conjugated diene per 100 parts by mass of the polypropylene resin and having a hydrogenation degree of at least 90%. The random copolymer has a hydrogenation degree of at least 90%. The styrene is present in the random copolymer in an amount of more than 5 mass % and less than 14 mass %. The conjugated diene consists of at least 60 mol % of one of a) 1, 2 or b) 3, 4 bonds on average. The styrene is present in the at least one of the a) block and b) random copolymer of styrene and a conjugated diene in an amount of more than 14 mass % and less than 50 mass %.
The thermoplastic elastomer composition may further include 5 to 250 parts by mass of an ethylene-xcex1-olefin copolymer rubber per 100 parts by mass of hydrogenated copolymer.
The invention is further directed to a skin formed by slush molding powder made from a thermoplastic elastomer composition as in any of the variations described above.